This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Increasingly the geographic position of mobile terminals is being used for various purposes. Many of these are for only user-related matters such as geo-tagging a photo or obtaining turn-by-turn navigation directions, but also the communications network has a need for UE position information which may require the UE to send its position information in an uplink location report. Regardless of the purpose, sometimes the UE does not have global positioning system (UPS) capability in which case the network can give assistance to the UE so the UE may more accurately determine its own position terrestrially. For example, the evolved Universal Terrestrial Radio Access Network (E-UTRAN, sometimes referred to as Long Term Evolution LTE) system provides that the UE can rely on transmissions or other information sent by the E-UTRAN eNodeB (eNB) to improve the accuracy of its terrestrial location. See for example 3GPP TS 36.305 v11.0.0 (2012-06); 36.355 v10.5.0 (2012-06) and 36.455 v10.3.0 (2012-06). These procedures also provide for the UE to listen to and measure neighbor cells for this terrestrial positioning, apart from its own serving cell.
The UE's location is kept in what is termed a location server, which can run several different positioning methods in parallel for a given UE. Two such positioning methods are termed the Observed Time Difference of Arrival (OTDOA) and the enhanced cell identity (ECID). FIG. 1A-C show the relevant E-UTRAN architecture, reproduced from FIGS. 5-1 and 5.1-1 of 3GPP TS 36.305 and FIG. 4.1.1-1 of 3GPP TS 36.355, respectively. The location server at FIG. 1A can be the Secure User Plane Location Platform (SLP) or the Enhanced Serving Mobile Location Center (E-SMLC).
The OTDOA is a downlink (DL) terrestrial positioning method specified in LTE Release 9 and is based on a UE performing DL Reference Signal Time Difference (RSTD) measurements using the Positioning Reference Signal (PRS) transmitted by the eNodeBs. There is also an uplink TDOA (UTDOA) method being standardized for Release 11. While the examples below focus on the former OTDOA method, it applies to the latter uplink TDOA method also. The PRS transmission from the eNodeBs is typically used for the UE to hear the neighbor eNodeBs and DL-TDOA accuracy is improved with more DL signals. For OTDOA the UE must first get a list of neighbor cells as assistance data from the location server to be able to perform the RSTD measurements, and it is typical to expect at least 3-4 to be transmitting within an area. The location server is responsible for selection of the neighbor cells to provide assistance data to the UE. The location server gets its information about neighbor cells either from operations and maintenance (O&M) or centralized self optimizing network (C-SON) configuration. Alternatively the location server can poll the individual eNodeBs to get the cell information. Specifics as to the signaling involved for the above information exchanges may be seen at 3GPP TS 36.305, 36.455 and 36.355 referenced above. The location server can provide assistance data to the UE or the UE can also request and get the assistance data from the location server. The location server polling the eNodeBs to get cell information for use as assistance data for the UE is done using LPPa protocol signaling, specifics which can be seen at 3GPP TS 36.455 referenced above.
The Enhanced Cell ID (E-CID) method is a serving cell based terrestrial positioning method that either uses DL measurements done by the UE of the serving cell (for example, reference signal received power RSRP, reference signal received quality RSRQ, UE receive-transmit Rx-Tx Time Difference) or UL measurements done by the serving eNodeB (for example, eNodeB Rx-Tx Time Difference and/or Angle of Arrival). In either of the above implementations for this method the serving eNodeB gets a LPPa Location Request message from the location server. Specifics as to this information exchange can also be seen at 3GPP TS 36.305 and 36.455 as referenced above.
A problem arises in that at least since Release 9 of the 3GPP standards for E-UTRAN, there is the possibility that some of the cells may be temporarily deactivated, and thus not available to support the UE's position measurements. The purpose of this deactivation feature is to save energy, for example where some base station was not transmitting during certain portions of the day (e.g., between midnight and 6 am) due to very low traffic volume. The time duration from a deactivation to reactivation may be for short time periods and vary widely as to when they occur during a day. So in practice there may not be a sufficient number of active neighbor eNodeBs for the UE to accurately fix its terrestrial location.
One possible solution is to not allow deactivation of nodes for energy savings, but this is wasteful when an eNodeB is active but essentially unused for extended periods of time. Another option is for the UE to determine its location with the serving cell and whatever other neighbor cells may be nearby and active, but the inaccuracies noted above with this approach may exceed certain mandates for location accuracy (for example, emergency calls for first responders) that are needed by other network functions.